1. Field of the Invention
The present invention relates generally to magnetic resonance imaging, and specifically to using motion-probing gradient pulses selected for isotropic diffusion weighting.
2. Background Discussion
Magnetic Resonance Imaging (MRI) is a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR). In MRI, imposing a strong main magnetic field (B0) on the nuclei polarizes nuclei in the volume of an object to be imaged. The nuclei are excited by a radio frequency (RF) signal at characteristic NMR (Larmor) frequencies. By spatially distorting localized magnetic fields surrounding the object and analyzing the resulting RF responses from the nuclei, a map or image of these responses as a function of their spatial location can be generated and displayed. An image of the nuclear responses provides a non-invasive view of an object's internal structures and of other properties.
It has been long known that by using motion-probing gradient (MPG) pulses, magnetic resonance (MR) images can be acquired with contrast sensitive to molecular self-diffusion. See, for example, Stejskal E, Tanner J, Spin diffusion measurements: spin echoes in the presence of time-dependent field gradient, J Chem Phys 42:288-292 (1965); Le Bihan D, Breton E, Lallemand D, Grenier P. Cabanis E, Laval-Jeantet M, MR imaging of introvoxel incoherent motions: application to diffusion and perfusion in neurologic disorders, Radiology 161:401-407 (1986).
In biological tissues, diffusion is in general anisotropic. By acquiring multiple data sets with varied MPG gradient direction and strength, diffusion anisotropy of water molecules in biological systems has been measured, imaged and used for study of some important pathophysiological properties, such as tissue fiber orientation. See, for example, Basser P J, Mattiello J, Le Bihan D, MR diffusion tensor spectroscopy and imaging, Biophys J 66:259-267 (1994); Xue R, van Zijl P C, Crain B J, Solaiyappan M, Mori S, In vivo three-dimensional reconstruction of rat brain axonal projections by diffusion tensor imaging, Magn Reson Med 42: 1123-1127 (1999). However, in many other cases, such as for diagnosis of stroke, strong anisotropy may instead impair the diagnosis by masking the underlying changes in the local apparent diffusion coefficient, which was found to be a better indicator for early detection of stroke. See, for example, Moseley M E, Cohen Y, Mintorovitch J, Chileuitt L, Shimizu H. Ducharczyk J, Wnedland M F, Weinstein P R, Early detection of regional cerebral ischemia in cats: comparison of diffusion- and T2-weighted MRI and spectroscopy, Magn Reson Med 14:330-346 (1990). For such applications weighting by the trace of the diffusion tensor, a.k.a. isotropic diffusion weighting, is preferable instead.
Isotropic diffusion weighting is usually achieved by combining multiple data sets from separate measurements with diffusion weighting in different directions. The simplest of these is to perform three separate measurements with diffusion weighting along three orthogonal directions in a laboratory reference frame. The images are combined to generate isotropic diffusion weighting. A problem encountered while using this approach is the long acquisition time needed in order to collect the required multiple data sets.
Recently, methods have been developed for acquiring images with isotropic diffusion weighting in a single shot. For example, an approach by Mori et al. uses combinations of bipolar gradients; as many as twelve pairs. See Mori S, van Zijl PCM, Diffusion weighting by the trace of the diffusion tensor within a single scan, Magn Reson Med 33.41-52 (1995) This approach, though simple to implement, is unfortunately very inefficient in producing diffusion weighting. If we define the efficiency as a ratio between the b value generated by the new gradient pattern to that by a simple pair of bipolar gradient pulses, the highest efficiency reported by this approach is about 0.188. This translates to need for increase in gradient by 230%
  (            1      0.188        )or lengthening of diffusion weighting time by 170%
  (            1      0.188        3    )in order to achieve the same diffusion weighting. Even using an optimized orthogonal gradient scheme, the highest efficiency achieved by the approach is only 0.25. See Chen, Z, Zhong, J, Optimized Orthogonal Gradient Technique For Fast Quantitative Diffusion MRI On A Clinical Scanner, U.S. Pat. No. 6,288,540 B1 (2001).
On the other hand, an approach by Wong et al. uses numerical techniques to manufacture gradient patterns that yield isotropic gradient weighting. See Wong E C, Cox R W, Song A W, Optimized isotropic diffusion weighting: Magn Reson Med 34:139-143 (1995). The advantage of their approach is that much higher efficiency for diffusion weighting can be achieved. The disadvantages are (1) the procedures used for manufacturing the gradient patterns are complex and require a lot of computation power; (2) the gradient patterns can only be applied in sequences with the exact same parameters assumed when the patterns are manufactured. When the sequence parameters are changed or when new sequences are used, the gradient patterns used need to be re-manufactured. Applying a complex procedure that demands high computational power on the fly is difficult in practice and may be entirely impractical.
There is, therefore, a great need for simple and efficient methods and procedures for manufacturing gradient patterns. Such procedures are to be used on the fly with real parameters used by the imaging sequence to produce gradient patterns with desired characteristics. The gradient patterns so generated should be easy to implement on any commercially available MR scanners with good efficiency for isotropic diffusion weighting.